Its designed to throw you off. They want to know how you react under stress. And most people are too proud to prepare a response. An answer along the lines of 'lack of technical expertise but that you have confidence in your ability to learn quickly' is probably the best thing you can say for most firms.

What's the worst job interview question??
I hate when they ask me "What's yoour weakness?"I never know what to say!!!

Apart from the obvious -'gee, where do I start?" ??
had this question in an interview yesterday. I said my greatest strength and greatest weakness are linked (eg Im a good communicator and love working thru probs in an open and honest team environment, but at same time i can be excessively sensitive to criticism) - shows some self awareness, with luck gets the panel thinking and diverts attention

pollies do it: answer the question u wanted them to ask, not the one they do ask

This isn't terribly common, but I had someone ask me "Would you rather fail to reject a hypothesis (when you should reject) or reject a hypothesis (when you shouldn't have)?"

My immediate reaction to myself was "that's meaningless," because this wasn't an arena where you really have to worry; you just care about the results one way or another. But not knowing what to say, and not wanting to say what I thought of this question, I just shrugged my shoulders and picked one with a shake of my head and, what I'm sure was a look of confusion.

I returned home afterwards and asked my former econometrics prof about it and her first response was "that question makes no sense."

any interviewer worth his salt wouldn't ask the question, but if they do, I'd hope they'd be able to look right through the answer given above. Sure you're supposed to turn a weakness into a strength, but it should be at least somewhat of a weakness. I've used...I tend to get too focused on the details and can struggle to see the big picture. For a details job, this is a good answer, if you're running a department, its not gonna fly

you should say something that is a legitimate weakness of yours, but one that you are actively trying to overcome and have had some success with overcoming it

the talker gave a personal example. he said he has issues with being impatient and being detailed enough. to overcome it he said at a former company he tried to take on projects that were super detail orientated and worked at it.

Q: "How would you go about testing the accuracy of a survey sample?" A: "I'm an econometrician, not a statistician" (mind you this was the 5-6th question along these lines) Result: I didn't get the job, but I didn't really want it anyways. Was using it as practice for jobs I actually wanted. I really wish companies would put half as much effort in developing solid job descriptions, as they do in developing these BS interview questions.

This isn't terribly common, but I had someone ask me "Would you rather fail to reject a hypothesis (when you should reject) or reject a hypothesis (when you shouldn't have)?"

My immediate reaction to myself was "that's meaningless," because this wasn't an arena where you really have to worry; you just care about the results one way or another. But not knowing what to say, and not wanting to say what I thought of this question, I just shrugged my shoulders and picked one with a shake of my head and, what I'm sure was a look of confusion.

I returned home afterwards and asked my former econometrics prof about it and her first response was "that question makes no sense."

That was positively my worst interview ever.

This is actually a quite interesting question. Nothing I'd ever expect to get in an interview .. but interesting nonetheless. For what it's worth .. this is asking "would you rather make a type I or type II error" .. and the correct answer is "it depends".